Metric coarse
Tap drill 6.75 mm for a M8 × 1.25 thread — major Ø 8 mm, pitch 1.25 mm (20.32 TPI).
Profile
| Designation | M8 × 1.25 |
|---|---|
| Major (outside) diameter | 8 mm · 0.315" |
| Pitch | 1.25 mm · 20.32 TPI |
| Pitch diameter | 7.188 mm · 0.283" |
| Minor diameter (external) | 6.466 mm · 0.2546" |
| Minor diameter (internal) | 6.647 mm · 0.2617" |
| Recommended tap drill | 6.75 mm · ≈77% thread |
| Clearance hole | Close 8.4 · Normal 9 · Loose 10 mm |
| Hex head (across flats) | 13 mm |
| Socket key (across flats) | 6 mm |
ISO 273 clearance holes (fine / medium / coarse).
Target preload is 75% of proof load; torque is estimated from T = K · F · d.
| Grade / class | Stress area | Preload | Torque |
|---|---|---|---|
| Class 8.8 | 36.6 mm² | 15.92 kN | 25.5 N·m |
| Class 10.9 | 36.6 mm² | 22.79 kN | 36.5 N·m |
| Class 12.9 | 36.6 mm² | 26.63 kN | 42.6 N·m |
For a M8 × 1.25 thread, drill the hole with a 6.75 mm tap drill (6.75 mm — the standard D − P rule) before tapping. That produces roughly 77% thread engagement, the usual target for a strong, tappable hole. The mating bolt has a major diameter of 8 mm and a pitch of 1.25 mm (20.32 TPI).
To pass a M8 × 1.25 bolt through a part rather than thread it, use a clearance hole: Close 8.4 · Normal 9 · Loose 10 mm.
All values are generated from the standard thread formulas (ISO 68-1 / ASME B1.1) and published standard tables — not hand-entered. See the full reference.
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